The first three hymns sung at the Divine Liturgy (Ps. 103; Ps. 146 and Only-begotten Son... the Beatitudes) are called Antiphons (steps) because they are sung in steps or stages by two Choirs singing opposite each other. Hence this type of singing in steps is called antiphonal.

Apostikha.

These are Stikhera accompanied by verses usually taken from the Psalms. The Apostikha is found at the end of Vespers and also at the end of Matins on ordinary weekdays.

Canon.

The Canon is a series of nine Canticles (or Odes) containing a number of Troparia in each, as well as a Theme Song (Irmos). The Canons are found at Matins, Compline, and certain other services in the Liturgical Cycle. Originally the nine Biblical Canticles were sung and short refrains inserted between each verse of the Canticle, but in time the Canticles themselves dropped out of general usage (except during Great Lent) and only the Theme Song (or Irmos), based on the theme of the original Canticle, and the refrains (now expanded) remained. The Second Ode is sung only as part of the Lenten Cycle and a tenth Biblical Canticle, the Magnificat is almost always sung after the Eighth Ode of the Canon.

Dogmatic.

This is a musical composition sung at Vespers and are so named because they speak of the dogma of the Two Natures of Christ.

Ikos.

This is a short composition that follows the Kontakion, between the Sixth and Seventh Odes of the Canon.

Irmos.

This is the Theme Song of each Ode of the Canon. The word Irmos means link, since originally the Troparia that followed it were sung in the same rhythm, and thus were linked to it.

Katavasia.

This is the concluding stanza of a Canticle of the Canon, so-called because, as the title implies (to go down), the Choir members came down into the center of the church to sing it. These are found after each Ode of the Canon on major Feasts and on ordinary days, the Irmos of the last Canon sung (there are usually several Canons sung together) is sung as Katavasia after Odes Three, Six, Eight and Nine.

Kathisma.

From the word kathizo I sit, these are selections from the Psalter, read at Vespers, Matins, and various other services, during which the Faithful are permitted to sit.

Kathisma Hymn.

These are short hymns sung after the Kathisma readings, during which the Faithful are permitted to sit (except for certain prescribed days). These are sometimes referred to as Sedalens or Sessional Hymns.

Kontakion.

The word means pole, since the Kontakion was originally a long poetic composition rolled up on a pole. Now only the brief preliminary stanza remains and is sung before the Ikos after the Sixth Ode of the Canon, at the Liturgy, Hours, and various other services.

Prokeimenon.

These are verses from the Psalter sung immediately before Scripture Lessons, primarily at Liturgy, Vespers and Matins. [Except for Feasts and during Great Lent, the Scripture Lessons themselves have generally fallen out of use at Vespers.] The Prokeimenon sung immediately before the Gospel Lesson is called the Alleluia.

Stikheron (Stikhera).

A Stikheron is a stanza sung between verses taken from the Psalms, primarily at Vespers (at Lord, I have called... and the Apostikha) and Matins (at the Apostikha).

Theotokion.

These are Troparia or Stikhera sung in honor of the Theotokos. On Wednesdays and Fridays, these Theotokia usually take the theme of the Theotokos at the Lord's Crucifixion, and thus are called Cross-Theotokia (or Stavro-Theotokia).

Troparion.

This is simply a short musical composition similar in length and style to the Kontakion. They are sung at the end of Vespers, after God is the Lord... and the Apostikha at Matins, at the Liturgy and other services.

Ypakoe.

This is a short Troparion sung at Matins on Great Feasts and Sundays.

Excerpt taken from "These Truths We Hold - The Holy Orthodox Church: Her Life and Teachings". Compiled and Edited by A Monk of St. Tikhon's Monastery. Copyright 1986 by the St. Tikhon's Seminary Press, South Canaan, Pennsylvania 18459.